The present invention relates to medical diagnostic X-ray imaging. In particular, the present invention relates to photon counting and energy discrimination to image selected and specific types of tissue or other structure.
Today, doctors and technicians commonly have access to very sophisticated medical diagnostic X-ray imaging devices. Typically during the operation of an X-ray imaging device, an X-ray source emits X-ray photons under very controlled circumstances. The X-ray photons travel through a region of interest (ROI) of a patient under examination and impinge upon a detector. In the past, X-ray imaging devices employed rudimentary film based detectors. However, recent developments have led to solid state detectors comprised of a grid of discrete detector elements that individually respond to exposure by X-ray photons. Regardless of the detector used, however, the goal remains the same, namely, to produce a clear resultant image of preselected structures of interest (e.g., specific types of tissues) within the ROI.
There is an inherent difficulty associated with producing a clear resultant image, however. In particular, because the X-ray photons travel through the entire patient, the image formed on the detector is a superposition of all the anatomic structures through which X-ray photons pass, including the preselected structures of interest. The superposition of anatomic structures is sometimes referred to as xe2x80x9canatomic noisexe2x80x9d. The effect of anatomic noise on the resultant image is to produce clutter, shadowing, and other obscuring effects that render the resultant image much less intelligible than the ideal clear resultant image.
Past attempts to reduce the effects of anatomic noise included, for example, xe2x80x9cdual-energyxe2x80x9d imaging. When employing dual-energy imaging, a doctor or technician acquired two images each with different average X-ray photon energies. Because different internal structures absorb different X-ray photon energies to different extents, it was possible to combine the two resultant images to suppress anatomic noise. Past dual-energy techniques typically proceeded in one of two ways.
A first approach used two stacked detectors. A single exposure then produced a first image in the first detector. Some X-ray photons continued through the first detector to impinge upon the second detector. The first and second detectors were designed to sense different average energies, thereby producing two images of the ROI corresponding to the two average X-ray photon energies. However, beyond the additional cost and complexity stemming from use of two stacked detectors, it was often difficult to obtain a large X-ray photon energy response separation between the two detectors. This caused the images produced by combining the two images using an algorithm designed to reduce anatomic noise to have poor contrast to noise ratio.
A second approach used a single detector and two exposures each with different average X-ray photon energy. Although this approach avoids the difficulties associated with the stacked detector, it suffers from its own problems. For example, patients often moved between exposures, thereby producing images of somewhat different internal structure between the two exposures. Furthermore, the X-ray source had to include additional circuitry to support selection of the specific X-ray photon energy to be produced.
A need has long existed in the industry for an imaging method that addresses the problems noted above and previously experienced.
A preferred embodiment of the present invention provides, for an X-ray imaging system, a method for energy dependent imaging of a region of interest. The method includes the step of exposing an X-ray detector to X-ray photons during an examination period, and separating the X-ray photons into two groups, those with energies above a selected energy threshold, and those with energies below a selected energy threshold.
The X-ray photons with energy above the threshold are counted to provide a first energy photon count, while the X-ray photons with energy below the threshold are counted to provide a second energy photon count. The method stores the first energy photon count and the second energy photon count in a memory as examination data, and produces an image by applying an image processing technique to the examination data.
Another preferred embodiment of the present invention provides an X-ray imaging system adapted for energy dependent imaging of a region of interest. The imaging system includes an X-ray detector responsive to X-ray photons during an examination period, an X-ray energy photon discriminator (with a variable energy threshold control input) coupled to the X-ray detector, and a memory connected to a processor.
The memory stores instructions for execution by the processor for reading a first energy photon count of X-ray photons above a selected energy threshold, and for reading a second energy photon count of X-ray photons below the selected energy threshold. The memory also includes instructions that direct the processor to store the first energy photon count and the second energy photon count in the memory as examination data, and apply an image processing technique to the examination data to produce an image.